Science with THESEUS

The Transient High Energy Sources and Early Universe Surveyor (THESEUS) will address multiple fundamental questions for modern cosmology and astrophysics. The primary scientific goals of the mission address the Early Universe ESA Cosmic Vision theme “How did the Universe originate and what is made of?” and, specifically, the sub-themes 4.1 Early Universe, 4.2 The Universe taking shape and 4.3 The evolving violent Universe. It will also have a relevant impact on the The Gravitaitonal Wave Universe (3.2) and “The Hot and Energetic Universe” themes. THESEUS will have unique capabilities to: a) Explore the Early Universe (the cosmic dawn and re-ionization era) by unveiling the Gamma-Ray Burst (GRB) population in the first billion years; and b) Perform an unprecedented deep monitoring of the soft X-ray transient Universe, thus providing a fundamental synergy with next-generation gravitational waves and neutrino detectors (multi-messenger astrophysics), as well as the large e.m. facilities of the next decade (ATHENA, E-ELT, SKA, CTA, LSST, etc.)

Because of their huge luminosities, mostly emitted in X- and gamma-rays, their redshift distribution, extending at least to z ~10 and their association with explosive death of massive stars, Gamma-Ray Bursts (GRBs) are unique and powerful tools for cosmology. In particular, GRBs represent a unique tool to study the early Universe up to the re-ionization era. To date there is no consensus on the dominant sources of re-ionization, and GRB progenitors and their hosts are very good representatives of the massive stars and star-forming galaxies that may have been responsible. A statistical sample of high–z GRBs (about 50 at z > 6) can provide fundamental information such as: measuring independently the cosmic star–formation rate, even beyond the limits of current and future galaxy surveys, the number density and properties of low-mass galaxies, the neutral hydrogen fraction, the escape fraction of UV photons from high-z galaxies. Even JWST and E-ELTs surveys, in the 2020s, will be not able to probe the faint end of the galaxy Luminosity Function at high redshifts (z>6-8). The first, metal–free stars (the so–called Pop III stars) can result in powerful GRBs (e.g. Meszaros+10). GRBs offer a powerful route to directly identify such elusive objects (even JWST will not be able to detect them directly) and study the galaxies in which they are hosted. Even indirectly, the role of Pop III stars in enriching the first galaxies with metals can be studied by looking to the absorption features of Pop II GRBs blowing out in a medium enriched by the first Pop III supernovae (Wang+12). More generally, high-z GRBs will allow the cosmic chemical evolution to be investigated at early times.

Besides high-redshift GRBs, THESEUS will detect and localize in the X-rays and promptly follow-up in the IR a large number of both transients and variable X-ray sources serendipitously during regular observations. These data will provide a wealth of unique science opportunities, by revealing the violent Universe as it occurs in real-time, through an all-sky X-ray smonitoring of extraordinary grasp and sensitivity carried out at high cadence. Here we emphasise the following primary objectives:

1. Provide real time trigger and accurate (~1-2 arcmin within a few seconds; ~1 arcsec within a few minutes) location of (long/short) GRBs and high-energy transients for follow-up with next-generation optical-NIR (E-ELT, JWST if still operating), radio (SKA), X-rays (ATHENA), TeV (CTA) or neutrino telescopes and identify electromagnetic counterpart of detections by next generation gravitational wave and neutrino detectors;

2. Discover new high-energy transient sources over the whole sky, including supernova shock break-outs, black hole tidal disruption events, magnetar flares, and monitor known X-ray sources, with high cadence observations.

By finding huge numbers of GRBs the survey will also permit unprecedented insights in the physics and progenitors of GRBs and their connection with peculiar core-collapse SNe, and substantially increase the detection rate and characterization of sub-energetic GRBs and X-Ray Flashes. The provision of a high cadence soft X-ray monitoring in the 2020s together with a 0.7m IRT in orbit will enable a strong synergy with transient phenomena observed with the Large Synoptic Survey Telescope (LSST) and the other large facilities that will be operating in the e.m. domain (E-ELT, SKA, CTA, JWST, ATHENA, …).